But true, four cores as near as I can tell. Manually setting the CPU affinity while running SuperPI I can stress each core individually, I don't know what other tests I can perform to prove its quad corelyness.

There is of course a down side, even at 1GHz Prime95 on all cores fails, but with another chip, well, who knows ?

As Wutske said, it is done in many situations, especially GPUs. Where do you think all those unlockable GPUs from years gone by came from, or how flashing a software BIOS magically upgrades a card's specs?

AMD has a certain number of defective CPUs they must sell as 3-core or 2-core parts. If their yields are TOO good and they lack sufficient defective chips, then they must cannibalize lower end chips in order to meet supplier demand.

They cannot simply decide to arbitrarily stop selling X3 CPUs just because they don't need to anymore. It requires channel notifications, EOL warnings, and the chance to allow suppliers to place final orders.

Why would an OEM buy a single, one-time batch of CPUs? It would not be cost effective for an OEM to design a product around a small quanitity of CPUs, or any other part just for one-time use.

Not saying they don't exist but I do not know of any "unofficial" OEM SKUs? OEM parts especially are required to have a specified level of market time before a CPU manufacturer can cease selling that SKU.